1. Field of the Invention
This invention relates to a rigid, fiber reinforced composite having improved impact and ballistic resistance, its precursor sub-assembly, and its method of manufacture.
2. Description of the Prior Art
Various constructions are known for composites used in impact and ballistic resistant articles such as helmets, panels, and vests. These composites display varying degrees of resistance to penetration by high speed impact from projectiles such as BB""s, bullets, shells, shrapnel, glass fragments and the like. U.S. Pat. Nos. 5,587,230; 5,552,208; 5,330,820; 5,196,252; 5,190,802; 5,187,023; 5,185,195; 5,175,040; 5,167,876; 5,165,989; 5,124,195; 5,112,667; 5,061,545; 5,006,390; 4,953,234; 4,916,000; 4,883,700; 4,820,568; 4,748,064; 4,737,402; 4,737,401; 4,681,792; 4,650,710; 4,623,574; 4,613,535; 4,584,347; 4,563,392; 4,543,286; 4,501,856; 4,457,985; and 4,403,012, PCT Publication No. WO 91/12136, and a 1984 publication of E.I DuPont De Nemours International S.A. entitled xe2x80x9cLightweight Composite Hard Armor Non Apparel Systems with T-963 3300 dtex DuPont Kevlar 29 Fibrexe2x80x9d describe ballistic resistant composites which include high strength fibers made from materials such as extended chain ultra-high molecular weight polyethylene and aramids. Such composites are said to be either flexible or rigid depending on the nature of their construction and the materials employed.
Ballistically resistant composites are formed from layers of fabrics or unidirectionally oriented sheets of fibers which are plied together. Where the individual plies are unidirectionally oriented fibers, the successive plies are rotated relative to one another, for example at angles of 0xc2x0/90xc2x0 or 0xc2x0/45xc2x0/90xc2x0/45xc2x0/0xc2x0 or at other angles. In previous processes, the individual plies of fabrics or fibers have been uncoated, or embedded in a polymeric matrix material which filled the void spaces between the fibers. If no matrix was present, the composite was inherently flexible. Bonding to a hard plate was required for rigidity. A contrasting type of construction is a composite consisting of fibers and a single major matrix material. To construct this type of rigid composite, individual plies were bonded together using heat and pressure to adhere the matrix in each ply, forming a bond between them, and fusing the whole into a unitary article.
The matrix resins employed in rigid composites were materials such as a vinyl ester resin or a styrene-butadiene block copolymer, and also mixtures of resins such as vinyl ester and diallyl phthlate or phenol formaldahyde and polyvinyl butyral. The rigidity, impact and ballistic qualities of the resulting composite depended to a high degree on the tensile modulus of the matrix resin. (Except as specifically noted, as used herein the terms tensile modulus and modulus mean the modulus of elasticity as measured by ASTM D638-94 for a matrix or interlayer material and ASTM D2256 for a fiber material.) For example, U.S. Pat. No. 4,623,574 discloses that fiber reinforced composites constructed with elastomeric matrices having tensile moduli less than about 6000 psi (41,300 kPa) have superior ballistic properties compared both to composites constructed with higher modulus resins, and also compared to the same fiber structure without a matrix. Unfortunately, low tensile modulus matrix resins, while yielding greater ballistic resistance, also yield lower rigidity composites. In certain applications, particularly those where a composite must function in both anti-ballistic and structural modes, there is needed a superior combination of ballistic resistance and rigidity.
The alternative approaches taken by prior art workers tend to maximize one property at the expense of the other, or to mix low modulus and high modulus materials in a single matrix to achieve a compromise in both properties. Thus, on the one hand, the 1984 DuPont publication cited above discloses the use of orthophthalic polyester resin, which is known to have a high tensile modulus of 800,000 psi (0.55 GPa). The other approach is exemplified by U.S. Pat. No. 4,403,012, which discloses a mixed high modulus phenolic and low modulus polyvinyl butyral resin matrix. A need exists for a rigid composite article that combines high rigidity with high ballistic and impact resistance.
Another problem faced by the skilled man seeking to manufacture rigid ballistic composites in an economical manner is the difficulty of bonding pre-impregnated sheets when the matrix resin is of high modulus. Useful methods of forming a pre-impregnated (prepreged) continuous web are disclosed and illustrated in U.S. Pat. No. 5,149,391 and U.S. Pat. No. 5,587,230, the disclosures of which are hereby specifically incorporated by reference thereto. These methods work well to produce continuous wound rolls of prepreged uniaxially oriented fiber sheet (unitape). The difficulty arises when the wound rolls of unitape are transferred to a cross-ply machine for construction of elementary two layer composites. Such machines are described in U.S. Pat. No. 5,173,138 and U.S. Pat. No. 5,766,725, the disclosures of which are hereby specifically incorporated by reference.
The cross-ply machine has the function of sequentially plying first and second rolls of uniaxially oriented fiber sheets with the longitudinal axis of a second ply rotated with respect to the longitudinal axis of the first ply, consolidating the two plies with heat and pressure, and winding up a continuous roll of cross-plied elementary composite.
Several problems are faced by the skilled man in cross-plying unitapes with high modulus matrix resins, particularly when the resin content is only about 25wt % or below. First, at relatively low temperatures below about 120xc2x0 F., the adhesion of the unitapes to each other is about the same as to a carrier web or release paper. This makes for difficulties in transferring unitape from the release paper to a second unitape sheet. Second, such resins require combinations of high temperatures, pressures and time in the cross-ply machine in order to consolidate sufficiently to wind up a continuous roll of product. Longer times mean lower production capacity. Higher temperatures can cause premature crosslinking of the matrix resin. Moreover, under these high temperature, pressure and time conditions, the unitape sheets adhere not only to each other, but to the cross-ply machine itself, causing frequent breakdowns and disruptions in production. A need exists for an improved method of producing composite articles with high rigidity and high impact and ballistic resistance using high modulus matrix resins.
The invention provides an impact resistant rigid composite comprising a plurality of fibrous layers. Each of the layers comprises a network of filaments having a tenacity equal to or greater than about 7 g/denier, a tensile modulus of at least about 150 g/denier, and an energy-to-break of at least about 8 J/g as measured by ASTM D2256. Every fibrous layer is in a matrix having a tensile modulus exceeding about 1xc3x97106 psi (0.69 Gpa) as measured by ASTM D638. An elastomeric layer is disposed between adjacent fibrous layers. The elastomer has a tensile modulus less than about 6000 psi (41,300 kPa) as measured by ASTM D638. The peel resistance between successive fibrous layers prior to curing of the matrix, when pressed at 66xc2x0 C. for 5 seconds at 332 psi (2,2290 kPa), is at least about 3 g/cm, preferably at least about 5 g/cm. The peel resistance between adjacent fibrous layers is measured by ASTM method D1876-95 where applicable or by the modification of D1876-95 described herein when the fibrous layers are uniaxially oriented.
The composites of the invention have high rigidity combined with superior ballistic properties. Notwithstanding these outstanding properties, it is contemplated that additional protection may be needed against projectiles designed to be armor piercing. To meet this objective, in another embodiment of the invention, the rigid composites disclosed above are bonded at one or both surfaces to a hard plate selected from the group consisting of metals and ceramics.
Yet another embodiment of the invention provides a sub-assembly precursor to an impact resistant rigid composite. Generally stated, the sub-assembly precursor comprises a plurality of fibrous layers. Each of the fibrous layers comprises a network of filaments having tenacity equal to or greater than about 7 g/denier, a tensile modulus of at least about 150 g/denier, and an energy-to-break of at least about 8 J/g. Every fibrous layer is in a thermosetting matrix, which when fully cured has a tensile modulus of at least about 1xc3x97106 psi (6895 MPa) as measured by ASTM D638. A layer of elastomer is disposed between the fibrous layers. The elastomer has a tensile modulus less than about 6000 psi (41,300 kPa) as measured by ASTM D638. The peel resistance between the two fibrous layers prior to curing of the matrix, when pressed at 66xc2x0 C. for 5 seconds at 332 psi (2,2290 kPa), is at least about 3 g/cm, more preferably at least about 5 g/cm.
The invention also provides a method for producing an impact resistant rigid composite, comprising the steps of: (a) forming first and second fibrous network sheets of high strength filaments having a tenacity equal to or greater than about 7 g/denier, a tensile modulus of at least about 150 g/denier, an energy-to-break of at least about 8 J/g; (b) impregnating each of the fibrous network sheets with a matrix resin having a tensile modulus when cured of at least about 1xc3x97106 psi (6895 MPa) as measured by ASTM D638; (c) applying to at least one surface of one of the fibrous network sheets a elastomeric material having a tensile modulus less than about 6000 psi (41,300 kPa) as measured by ASTM D638; (d) laying a first fibrous network sheet onto the surface of a second fibrous network sheet with the elastomeric material therebetween; (e) consolidating the first and second fibrous network sheets into two layer composite; (f) plying a plurality of two layer composites one upon another; and (g) consolidating the plurality of two layer composites and fully curing the matrix resin by means of heat and pressure.
It has been found that incorporation of a low modulus elastomeric layer between the rigid fibrous layers markedly improves the impact and anti-ballistic properties of the composite. Surprisingly, the improved properties are obtained without effecting either the glass transition temperature of the matrix or the rigidity of the composite. Articles made in accordance with this invention exhibit improved utility for applications requiring impact and ballistic resistance combined with high rigidity. Representative of such articles are helmets, shields, breastplates, panels and structural members of helicopters and aircraft.
The efficiency and economy of manufacture realized when producing composites of the invention are further improved by the incorporation of low modulus elastomeric layers. Lower temperatures and pressures can be utilized on the cross-ply machinery and sticking problems experienced in continuous operation are avoided.